Disagree. It was Yasu's god who said about Akira's Mom at the Sensei's knowledge competition and, given that God always answered honestly (this is how he most likely called Akira's true age, which confused Yasu), there is no point in not believing him. I think you are too obsessed with the idea that "the source of all evil is the Sekai alone".We don't know that Saki doesn't like her son. We know that Akira THINKS she doesn't.
One common tactic used by abusers is to isolate their victims from friends and family. This is normally done well before any abuse actually begins, when the victim still fully trusts their future abuser. If there's the slightest strain in the relationship at all, an abuser will seize upon it to drive a permanent wedge between them. A teacher has the opportunity to get a LOT of information if they wanted to do this to one of their students. A teacher dating that student's brother has literally EVERY piece of ammunition possible.
Because of this, we have to take Akira's limited memories of his mother with a grain of salt. That well was very likely poisoned by Sekai so thoroughly that he's not even aware of how untrue it is.